Question: Let $f : \mathbb{R} \to \mathbb{R}$ be a function such that
\[f(f(x) + y) = f(x^2 - y) + 4f(x) y\]for all real numbers $x$ and $y.$

Let $n$ be the number of possible values of $f(3),$ and let $s$ be the sum of all possible values of $f(3).$  Find $n \times s.$
Answer: Let $y = \frac{x^2 - f(x)}{2}.$  Then
\[f \left( f(x) + \frac{x^2 - f(x)}{2} \right) = f \left( x^2 - \frac{x^2 - f(x)}{2} \right) + 4f(x) \cdot \frac{x^2 - f(x)}{2}.\]Simplifying, we get
\[f \left( \frac{x^2 + f(x)}{2} \right) = f \left( \frac{x^2 + f(x)}{2} \right) + 2f(x) (x^2 - f(x)),\]so $f(x) (x^2 - f(x)) = 0.$  This tells us that for each individual value of $x,$ either $f(x) = 0$ or $f(x) = x^2.$  (Note that we cannot conclude that the only solutions are $f(x) = 0$ or $f(x) = x^2.$)  Note that in either case, $f(0) = 0.$

We can verify that the function $f(x) = x^2$ is a solution.  Suppose there exists a nonzero value $a$ such that $f(a) \neq a^2.$  Then $f(a) = 0.$  Setting $x = 0$ in the given functional equation, we get
\[f(y) = f(-y).\]In other words, $f$ is even.

Setting $x = a$ in the given functional equation, we get
\[f(y) = f(a^2 - y).\]Replacing $y$ with $-y,$ we get $f(-y) = f(a^2 + y).$  Hence,
\[f(y) = f(y + a^2)\]for all values of $y.$

Setting $y = a^2$ in the given functional equation, we get
\[f(f(x) + a^2) = f(x^2 - a^2) + 4a^2 f(x).\]We know $f(f(x) + a^2) = f(f(x))$ and $f(x^2 - a^2) = f(x^2),$ so
\[f(f(x)) = f(x^2) + 4a^2 f(x). \quad (*)\]Setting $y = 0$ in the given functional equation, we get
\[f(f(x)) = f(x^2).\]Comparing this equation to $(*),$ we see that $4a^2 f(x) = 0$ for all values of $x,$ which means $f(x) = 0$ for all $x.$  We see that this function satisfies the given functional equation.

Thus, there are two functions that work, namely $f(x) = 0$ and $f(x) = x^2.$  This means $n = 2$ and $s = 0 + 9 = 9,$ so $n \times s = \boxed{18}.$